Samples of analyzers for clinical chemistry are usually serum or plasma that is separated from a blood sample by centrifugation. Some changes may occur in the sample during sampling, centrifugation and processing of the sample. These changes, for example hemolysis, alter the composition of the sample and compounds that may disturb the analysis of the sample may be generated. The serum or plasma sample may also contain higher than normal amounts of substances that disturb the measurement. Higher concentration of hemoglobin, bilirubine or lipemia may be caused by the medical condition of the patient. These components have strong colors or they are highly turbide. If these components are present in a concentration that is higher than normally, the analysis of the sample may be seriously disturbed and the analysis results distorted.
Faulty samples can be usually discovered visually because of anomalous color or turbidity. Of course, an accurate estimate of the concentration cannot be achieved by visual control. If several, disturbing components are present simultaneously, the valuation of the sample is even more complicated. By using visual control, only rejection of a faulty sample is possible. The defects in color of a sample can be detected only after centrifugation. Normally the samples have been checked before loading them to an analyzer. However, laboratory automation has made it possible to perform the centrifugation and feeding of the samples to the analyzers automatically. Therefore no visual control of the samples is used and faulty samples may enter the analyzer and cause wrong results. Since the results are often used in diagnosing diseases, it is extremely important to prevent any faulty results that may lead to wrong treatment or medication.
The main causes for disturbed analyze are hemoglobin, bilirubin and lipids. Additionally, medication that color the serum or plasma as well as biliverdine may cause difficulties in measurement. In analyzers that are presently on market, only three first mentioned substances are observed. The typical concentration of bilirubin is up to 500 μmol/l and values of 100-200 μmol/l are common, the highest values being 1000 mol/l. Lipeamia is present in amounts of 1-2 g/l, but even values of 10 g/l may exist. The amount of hemoglobin is in the range of 2-3 g/l, 5 g/l is consider to be rather high value and measuring range is usually limited to 10 g/l.
One way to measure bilirubin, hemolyse and lipemia is to use a diluted sample that is measured on three different wavelengths. The sample is diluted by water or saline and measured on wavelengths of 405, 425 and 700 nm. Measurement is done by direct absorbance measurement or via reflection. The sample is then classified in categories whereby a HIL-index is obtained. Every category corresponds with a certain concentration range. When a HIL-disturbance limit is exceeded an alarm is initiated. The wavelengths used may vary, but the handling of results is in basic similar in different apparatuses.
By studying the spectrum of a sample, it is clear that this method is reliable on samples that contain only one HIL-component. The spectrums of different components do overlap, whereby it is possible by this method to detect that something is wrong with the sample, but it is not possible to detect what are the components causing the problem or what are the concentrations of different components. If simple detection of a faulty sample is satisfactory, this method may well be used. If the concentration of every component is to be measured, the situation is more difficult. A raised concentration of lipemia raises the absorbance on all wavelengths whereby even bilirubin and hemoglobin obtain erroneous high values even though the concentration of these substance were on an acceptable level. Especially the interference of bilirubin and hemoglobin is difficult to control. Further weaknesses of this method is that it consumes some of the sample (5-10 μl) and adds measuring steps for each sample thereby reducing the analyzing speed and capacity of the analyzer.
An alternate way to detect HIL-components is to obtain an absorbance spectrum for the whole sample over wavelengths of 300-1000 nm. The sample may be diluted or undiluted and the measurement can be done directly by absorption or indirectly by reflection. The spectrum is detected either on a sample tube or in an intermediate storage (at the end of a pipette, for example). Various calculation algorithms may be used for studying the spectrum and reasonable good estimation of the concentration of different HIL-substances can be obtained. The benefit of this method is that no sample has to be wasted and the measurement does not spend the capacity of the analyzer. The method requires anyhow an extra spectrometer and transfer means for the sample. This raises the costs and the complexity of the apparatus considerably.
Abovementioned methods have been described in US 2002/0089669, U.S. Pat. No. 6,353,471, US 2001/0004256 and US 2002/0110487.